It seems a bit odd that a power supply from someone like DEC of that era would be designed to depend so critically on the absolute value of a rail used for startup purposes.
The output of the replacement 7812 varying between 12.2V and 12.4V suggest that the 7812 itself could be faulty. Or maybe it could be oscillating due to a failure in the smoothing/reservoir/decoupling capacitors on either side of it, especially the 10uF and 2.2uF capacitors connected to it's output? It might be useful to see what Vstart looks like with an oscilloscope. If the value of Vstart is really critical enough to change the state of E3d, I would suggest checking the resistors connected to the inputs and output of E3d, especially the 360k resistor. Assuming it is getting a reasonable power supply on pins 3 and 12 (whose value should not be critical), the output of E3d should depend solely on the relationship between the voltages at it's two inputs. If the output does not correspond with what the inputs say it should be, then the comparator itself is probably faulty. Regards, Peter Coghlan. > > I need to order higher tolerance ones. However, I was wondering, would a > sensible strategy be to go through the ones that I have and find one that > outputs a voltage closer to the spec? Or would that just be storing up > trouble for the future? > > Regards > > Rob > > > -----Original Message----- > > From: Rob Jarratt <robert.jarr...@ntlworld.com> > > Sent: Sunday, April 30, 2023 9:35 PM > > To: 'Wayne S' <wayne.su...@hotmail.com> > > Cc: r...@jarratt.me.uk; 'General Discussion: On-Topic and Off-Topic Posts' > > <cctalk@classiccmp.org> > > Subject: RE: [cctalk] Re: Rainbow H7842 PSU Fault > > > > Looks like the one I used is an L7812CV, which has a looser tolerance. I am > > going to have to order some tighter tolerance parts. > > > > > -----Original Message----- > > > From: Wayne S <wayne.su...@hotmail.com> > > > Sent: Sunday, April 30, 2023 7:42 PM > > > To: Wayne S <wayne.su...@hotmail.com> > > > Cc: r...@jarratt.me.uk; General Discussion: On-Topic and Off-Topic > > > Posts <cctalk@classiccmp.org>; Rob Jarratt > > > <robert.jarr...@ntlworld.com> > > > Subject: Re: [cctalk] Re: Rainbow H7842 PSU Fault > > > > > > The CT version is 4%, the ACT is 2%. > > > > > > Sent from my iPhone > > > > > > > On Apr 30, 2023, at 11:37, Wayne S <wayne.su...@hotmail.com> wrote: > > > > > > > > Spec sheet for the 7812 shows 2 versions - 4% and 2% tolerances. So > > > > the > > > voltages your measuring are within the 4% part. Maybe you do need the > > > 2% one. > > > > > > > > Sent from my iPhone > > > > > > > >> On Apr 30, 2023, at 11:26, Rob Jarratt via cctalk > > > >> <cctalk@classiccmp.org> > > > wrote: > > > >> > > > >> I think I have found a possible cause, but not too sure what to do > > > >> about it if I have. > > > >> > > > >> I had to replace the 7812 regulator that supplies Vstart. The new > > > >> one has an output that seems to vary between 12.2 and 12.4V. The > > > >> 7812 on the good PSU outputs a steady 12.1V. I think that is enough > > > >> to cause the E3d comparator to turn the comparator off and allow > > > >> its output to > > > float high. > > > >> > > > >> I suspect I may have used one with a wide tolerance rather than a > > > >> narrow one, or just have a bad one. It should be a genuine part as > > > >> it came from CPC Farnell. Not quite sure why the output varies on > > > >> the bad > > > one though. > > > >> > > > >> Regards > > > >> > > > >> Rob > > > >> > > > >> > > > >>
